There are numerous examples in the literature of processes that describe the preparation of fluid separation hollow fiber membranes. These membranes can be either anisotropic, most frequently asymmetric, or isotropic, and may be useful for a variety of fluid separations, including gas separations. For example, I. Cabasso describes the procedures for preparing porous polysulfone hollow fibers in "Hollow Fiber Membranes", Kirk Othmer: Encyclopedia of Chemical Technology, 12, 3rd Edition, 492-519 (1980) and "Membranes", Encyclopedia of Polymer Science and Engineering, 9, 2nd Edition, 509-579, (1987).
Among the methods most frequently used for preparation of asymmetric membranes is the phase inversion process. The preparation of asymmetric ("skinned") membranes from polymer solutions by phase inversion methods usually involves the stages of: (1) casting the polymer solution or "dope", (2) evaporation by exposure of the cast solution to the atmosphere, (3) precipitation of the solution in coagulation media and leaching out solvents, and optionally, (4) the annealing of the membrane. Although preparation of asymmetric membranes by direct coagulation without the evaporation step is known, most industrial procedures do include this step. The importance of the evaporation step is well recognized in the art but is also highly empirical with optimal parameters such as evaporation temperature, duration of the evaporation step, etc. being determined experimentally for a particular polymer/solvent casting composition.
Preparation of anisotropic membranes present a challenge in that the morphological requirements for optimizing each section of the membrane are not necessarily the same. For example, the coagulation liquid required to form optimal surface porosity and the one required for formation of optimal sub-surface structure may be different. This means that when a single coagulant is used, it represents a compromise and neither structure is optimized or alternatively, the nascent fiber must be passed through a series of coagulant baths. An example of the latter approach can be found in J. A. van't Hof et al., "Preparation Of Asymmetric Gas Separation Membranes With High Selectivity By A Dual-Bath Coagulation Method", J. Membrane Sci., 70 (1992), pages 17-30, in which two successive non-solvent baths are used wherein the first bath initiates the formation of the exterior layer and the second bath completes the solidification process. Care must be taken in practicing this method, however, because the coagulant baths may be composed of expensive, flammable and/or toxic liquids which can escape to the surrounding environment. In addition, the fiber must pass through an air gap between the first and second non-solvent baths despite the fact that it has not been fully coagulated.
A procedure for preparing fibers from polymer solutions at subatmospheric pressure is disclosed by Stoy et al. in U.S. Pat. No. 3,842,151. According to the invention disclosed, a polymer solution is extruded through a spinneret into a tube or shaft whose upper end is sealed against gas flow by a lid connected with the spinneret, and its lower end is placed below the level of a coagulation bath open to the atmosphere. The pressure within the tube between the spinneret and the level of the coagulation bath is maintained lower than the pressure outside the tube or shaft which causes the level of the coagulation liquid to be higher in the shaft than in the outer coagulation bath. In the spinning process, the polymer solution is extruded through the spinneret into the gaseous atmosphere above the coagulation bath in the shaft, which is maintained at subatmospheric pressure. The fibers then enter the coagulation bath and after passing through the bath are collected. Vacuum means are provided to maintain the level of the coagulation bath at the desired height and provision is also made to introduce and remove gaseous medium from the shaft area between the spinneret and the top of the coagulation bath if desired.
Repetti et al. in U.S. Pat. No. 4,915,888 employ a vertical column of quench liquid which is supported by a vacuum such that the quench liquid is in contact with the spinneret outlet. This type of apparatus is conveniently used to provide a long coagulation path for nascent dope streams which are used to make microporous nylon hollow fibers.
B. Bikson et al. in U.S. Pat. No. 5,181,940 describe the use of a vacuum spinning apparatus to produce highly asymmetric hollow fiber membranes with improved fluid or gas separation properties. The spinning dope is extruded into a zone maintained at a reduced pressure as compared to the exterior of the spinning chamber and the cast dope is subjected to controlled evaporation under the subatmospheric pressure prior to being introduced into the homogeneous liquid which acts as the coagulation medium.